The present invention relates to a clutch apparatus for an antenna which is raised and lowered by a driving force from a motor.
Various clutch apparatuses of this type have been conventionally available. As disclosed in Japanese Utility Model Publication No. 48-7623, the most popular type of apparatus comprises a driving clutch plate rotated by a driving force from a motor and a driven clutch plate coupled to an antenna. The driving clutch plate is brought into tight contact with the driven clutch plate. In the case when the antenna is raised or lowered, the driving and driven clutch plates are rotated together, and the antenna is thereby raised or lowered by the driving force of the driven clutch plate. When the telescopic operation of the antenna is completed, the driven clutch plate is stopped, and the driving clutch plate is kept rotating in a slipping state with respect to the driven clutch plate. Thereafter, a limit switch or the like is operated, thereby stopping rotation of the driving clutch plate. Note that a plurality of recesses and projections are formed on a contact surface between the driving and driven clutch plates in order to obtain a desired clutch force.
In the clutch apparatus having the above arrangement, the driving force transmitted from the motor to the antenna mainly depends upon a frictional force between the driving and driven clutch plates. For this reason, sufficiently large contact force must be applied between the driving and driven clutch plates. For example, assume that, when the antenna is raised or lowered, a load of about 7.5 kg is required to drive the driven clutch plate. In this case, when the recesses and projections are formed on the contact surfaces of the two clutch plates, a contact force of about 20 kg must be applied. On the other hand, in the case of clutch plates having no recesses and projections on their contact surfaces, a contact force of about 30 to 50 kg must be applied. As a result, in order to maintain a sufficient contact force, at least one large and strong spring is needed, resulting in a bulky apparatus. Furthermore, when the driven clutch stops, that is, when the telescopic operation of the antenna is completed, the driving clutch plate slips, thereby generating loud noise.